The present invention relates to an open-end bladder for the vulcanization of a pneumatic tire, particularly to an open-end bladder which is uniformly expanded to uniformly apply pressure to the entire inside surface of an unvulcanized tire and keep the tire in a prescribed position on the inside surface of vulcanizing molds in preshaping the unvulcanized tire in conformity with the molds.
A conventional bladder is used to press an unvulcanized tire on vulcanizing molds to vulcanize the tire through the use of a fluid of high temperature and pressure to form the beads, side walls and tread of the tire, whether the tire is a bias-ply tire, a radial-ply tire or the other. During the period of time from the insertion of the bladder into the unvulcanized tire to the vulcanization thereof, the bladder supports the unvulcanized tire on the inside surface thereof and expands the tire to properly hold the tire on the inside surfaces of the vulcanizing molds to preshape the tire. The vulcanizing molds are completely closed on each other at timing set relative to the end of the preshaping. When the molds are completely closed on each other, the heating fluids such as steam is introduced into the molds and the bladder.
At the time of the preshaping, an inactive gas such as air, carbon dioxide and nitrogen is introduced as an expanding medium into the bladder. At the time of the vulcanization, the heating fluid such as steam is introduced into the bladder to replace the expanding medium therein. The pressure of the heating fluid introduced into the bladder for the vulcanization is higher than that of the expanding medium introduced into the bladder for the preshaping and acts to press the unvulcanized tire on each of the tire tread patterning projections and recesses of the inside surfaces of the vulcanizing molds. In other words, the heating fluid of high pressure and temperature is introduced into the bladder to expand the bladder to bring the unvulcanized tire into tight contact with the inside surfaces of the vulcanizing molds at the time of the vulcanization of the unvulcanized tire.
Such bladders are classified into the closed-end type in which the lower end of the bladder is open, and the open-end type in which the upper and lower ends of the bladder are open.
FIG. 1 shows a conventional open-end bladder 2. Since the bladder 2 is engaged with vulcanizing molds not shown in the drawing, the metal-clamped portions 4a and 4b of the bladder are provided with a large thickness. However, the body 6 of the bladder, which corresponds to the side walls and tread of a tire, is provided with a uniform thickness. The outside surface of the body 6 is usually provided with air release grooves. At 8a and 8b in FIG. 1 are openings which extend through the bladder 2 in the axial direction thereof.
A series of steps to be taken to preshape the tire by using the conventional bladder 2 are described now with reference to FIGS. 2A, 2B and 2C. A disk-shaped member 14 is secured at the center of a lower mold 12a which is for vulcanizing the unvulcanized tire 10. A center rod 16 extends from a cylinder (not shown in the drawings) through the central portion of the secured member 14 so that the center rod can be moved vertically. The secured member 14 has a fluid passage (not shown in the drawings) for introducing a preshaping pressure fluid such as air into the bladder 2. A disk-shaped plate 18 is engaged with the secured member 14. An attaching metal 20, whose top slightly sloped down toward the center thereof, is secured to the top of the plate 18. A disk-shaped plate, whih slightly slopes down toward the periphery thereof, is engaged with the center rod 16 near the upper end thereof. An attaching metal 21 is secured to the peripheral edge of the plate 19. The metal-clamped portion 4a of the bladder 2 is pinched between the plate 18 and the attaching metal 20, while the other metal-clamped portion 4b of the bladder is engaged with the attaching metal 21. The bladder 2 is thus set. At that time, an upper mold 12b is located over the lower mold 12a, and the bladder 2 keeps its original form, as shown in FIG. 2A. After that, the cylinder is driven to move up the center rod 16 to elongate the bladder 2, and the interior of the bladder is evacuated through an air passage not shown in the drawings. As a result, the middle portion of the bladder 2 is concaved so that the bladder is prevented from coming into contact with the unvulcanized tire 10. The unvulcanized tire 10 hung from the diameter-reducible plate 24 of a loader 22 is then moved down so that the bead 10a of the tire is positioned on the circumferential fitting portion 26 of the lower mold 12a. The center rod 16 is thereafter moved down. Air is introduced into the bladder 2 through the air passage at timing set relative to the moving-down of the center rod 16, so that the bladder is expanded into pressure contact with the inside surface of the unvulcanized tire 10. The upper mold 12b is then moved down so that the upper and the lower molds 12b and 12a enclose the unvulcanized tire 10. A vulcanizing fluid is then introduced into the bladder 2.
However, since the bladder 2 has the above-described form, a pressure difference arises between a portion of each of the bladder 2 and the unvulcanized tire 10 and another portion of each of them at the time of the introduction of the expanding gas into the bladder, and the time which it takes to preshape the tire differs from portion to portion, as shown in FIGS. 3A and 3B. The reference symbols indicating the portions of the tire 10 in FIG. 3A correspond to those in FIG. 3B. It is understood from FIGS. 3A and 3B that the pressure contact point of the bladder 2 on the tire 10 moves from the bead of the tire toward the center of the tread thereof as the time of the preshaping passes. The pressure on the tire 10 much differs from portion to portion so that the pressure difference arises between the portions of the tire at the end of the preshaping, as shown in FIG. 3B.
The conventional bladder 2 has problems described from now on. Since the bladder 2 has the above-described form, air is hermetically enclosed it between the outside surface of the bladder and the inside surface of the unvulcanized tire 10, as shown in FIG. 2C, when the expanding medium is introduced into the bladder. As a result, the outside surface of the vulcanized tire is made rugged at the place of the hermetic enclosure of the air to make the vulcanized tire defective. In a tire forming process in which the components of the tire are piled up as layers, the hermetically enclosed air is not released but remains in the tire because the beads of the tire are sealed in the preshaping of the tire. In other words, the air is enclosed in between the rubber, cords and the like of the vulcanized tire, which not only makes the tire into a defective product but also is likely to lead to an accident if the tire is put into practical use. Because of such problems, it is inevitable that more strict quality control is done and the process of production of the tire is complicated. Besides, since the bladder 2 is expanded into pressure contact with the portions of the inside surface of the unvulcanized tire 10 under different levels of pressure thereon during the preshaping the tire, the bladder is sometimes improperly positioned in contact with the inside surface of the unvulcanized tire. For that reason, the inside surface of the tire is wrinkled, and the tire is unstably preshaped to improperly form each bead of the tire to make the tire defective.